Retarding field electron spectrometer

ABSTRACT

An electron spectrometer suitable for use in Electron Spectroscopy Chemical Analysis, or in Auger Electron Spectroscopy employs either an X-ray source or an electron beam source for exciting electrons from the surface of a test sample in vacuum. A substantial free drift path through a wide spherical angle is provided for ejected electrons, together with a pair of spaced apart spherically shaped retarding potential grids having a plurality of fringing field correction rings therebetween and the situs of a test sample as the center of their curvature, a cylindrical pot-shaped general electron integral current collector anode lined with a negatively biased screen grid secondary electron repeller, and a small conically shaped selective electron, differential current collector insulatingly passed through a central coaxial opening in the anode and connected to an electron multiplier output. The entire structure is enclosed in a vacuum within an external ferromagnetic casing which provides both electrostatic and electromagnetic shielding. Additionally an external magnetic shield is employed, or Helmholtz coils may be used, to reduce stray magnetic fields to not more than 10 milligauss. The device accommodates relatively large test samples, employs no input or output slits and hence provides a substantially greater signal level than prior spectrometers, and is capable of high resolution of the order of 0.04 percent.

United States Patent Rigden et al.

[ 51 Aug. 1,1972

[54] RETARDING FIELD ELECTRON SPECTROMETER [72] Inventors: Jameson Dane Rigden, Westport; David A. Huchital, Trumbull, both of Conn.

[73 Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

[22] Filed: Oct. 24, 1969 21 Appl. No.: 870,490

[52] US. Cl.....250/49.5 A, 250/495 E, 250/49.5 PE [51] Int. Cl ..H0lj 37/26 [58] Field of Search .....250/41.9 D, 49.5 A, 49.5 PE,

[56] References Cited UNITED STATES PATENTS 2/1957 Lofgren et a1. ..250/4l.9

OTHER PUBLICATIONS Primary Examiner-William F. Lindquist AttorneyEdward R. Hyde, Jr.

[ ABSTRACT An electron spectrometer suitable for use in Electron Spectroscopy Chemical Analysis, or in Auger Electron Spectroscopy employs either an X-ray source or an electron beam source for exciting electrons from the surface of a test sample in vacuum. A substantial free drift path through a wide spherical angle is provided for ejected electrons, together with a pair of spaced apart spherically shaped retarding potential grids having a plurality of fringing field correction rings therebetween and the situs of a test sample as the center of their curvature, a cylindrical pot-shaped general electron integral current collector anode lined with a negatively biased screen grid secondary electron repeller, and a small conically shaped selective electron, differential current collector insulatingly passed through a central coaxial opening in the anode and connected to an electron multiplier output. The entire structure is enclosed in a vacuum within an external ferromagnetic casing which provides both electrostatic and electromagnetic shielding. Additionally an external magnetic shield is employed, or Helmholtz coils may be used, to reduce stray magnetic fields to not more than 10 milligauss. The device accommodates relatively large test samples, employs no input or output slits and hence provides a substantially greater signal level than prior spectrometers, and is capable of high resolution of the order of 0.04 percent.

22 Claims, 8 Drawing Figures United States Patent [151 3,681,600

Rigden et al. [4 Aug. 1, 1972 PATENTEDAuc I I972 SHEET 1 OF 4 VAC INVENTORS' Jameson M72 35 4 D UL -HZZ ATTORNEY PATENTED 3.681.600

sum 2 BF 4 PATENTEUAUB 1 I972 sum 3 or 4 1 RETARDING FIELD ELECTRON SPECTROMETER BACKGROUND OF THE INVENTION The art of electron spectroscopy is based upon the recognition that all matter is composed of atoms, each of which consists of a central core or nucleus carrying a positive charge, and an atmosphere of electrons arranged in concentric shells surrounding the nucleus to neutralize the nuclear charge. The outermost electrons are those which give the atom its chemical nature and which are shared with other atoms to make molecules or solids. These also are the electrons which may be most easily removed to leave the atom in a charged or ionized state.

In any atom, the electrons arearranged in different energy levels and a convenient way of visualizing these levels is to consider the energy required to remove the electron from the atom. The outer electrons are removed with comparatively low energies while those in deeper shells require more energy because they are closer to the positive charge on the nucleus and, thus, more strongly attached.

One way in which an atom, excited by the loss of an electron from a deep shell, may return to normal is to fill the vacancy with one of its outer electrons. However, there is still an excess of energy to be dissipated. Often, this excess is enough to eject another of the outer electrons entirely. These emitted electrons, resulting indirectly from excitation of atoms in their inner shells, are known as Auger electrons. For light atoms, the probability of Auger electron emission is greater than the probability of X-ray photon emission. For the heavier elements, the reverse is the case.

The technique of Electron Spectroscopy Chemical Analysis (ESCA) as well as the technique of Auger Electron Spectroscopy (AES) depends upon determining the number or density of electrons emitted from a sample under test at each of several different energy levels. One directs a monoenergetic stream of X-ray photons for ESCA, or a primary electron beam for AES, of sufficient energy to eject some electrons from a molecule of the sample. The resulting ejected electrons will have a variety of energies. ln ESCA the observed energy for each is the difference between the energy of the incident particle, photon or electron, and the energy required to just remove the electron from whatever state it existed in the atom.

By recording, or displaying on an oscilloscope, the electron density at progressively changing energy levels a curve is plotted on which the appearance and spacing between distinctive peaks of electron counts signify the elemental composition of the molecular structure under examination. The technique of ESCA requires a relatively high resolution, of the order of 0.05 percent to 0.] percent.

STATE OF THE PRIOR ART The pioneer in the development and application of ESCA is Kai Siegbahn of Uppsala University who published his work in December 1965 entitled Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy. Siegbahn employs very large, costly and complicated spectrometers originally built for basic nuclear research, and having the potential of a higher degree of resolution than is required for electronic chemical analysis.

The most familiar electron spectrometers used in the ESCA technique operate by imposing a slit/baffle system between the electron source and the energy dispersing means which may be either electrostatic or magnetic. The narrow beam of electrons which passes through the entrance slit is bent into a curved path by the force of the dispersing means. In this way the lower energy electrons are caused to travel through a tighter curve than the higher energy electrons and thus the electrons of different energy levels are separated. By positioning a detector to pick up only a portion of the dispersed beam the current flow at a selected energy level can be determined. Ordinarily an exit slit is positioned over the detector to prevent stray adjacent electrons from entering. The resolution of such devices is inversely related to the area of the irradiated sample, the angular divergence of the current passing through the entrance slit, and the fraction of the dispersed electron flux which is accepted by the detector. In theory the resolution of these devices can be increased indefinitely by reducing the size of the entrance and exit slit openings. However, when the electron source operates at a constant intensity, as it must for ESCA, a major portion of the signal is lost by such a procedure, and the resulting spectra are very dim, requiring long observations for their resolution.

Spectrometers based upon the retarding potential difierence technique have been used for many years for various problems in charged particle energy spectroscopy. The paper by M. Caulton published in volume 26 of the RCA Review of 1965 is a thorough summary of the art to that time. A particular configuration of the retarding potential difference (RPD) spectrometer that is appealing for this application consists of a pair of grids formed into spherical sectors between which the retarding field is applied. The source of electrons is located at the center of the system and the collector is a solid spherical element located outside the grids. This analyzer has the advantage that no slits are necessary so that a very large fraction of the electrons emitted by the sample are collected.

There are, however, a number of very basic and significant reasons why a simple spherical RPD spectrometer is not applicable to the ESCA technique. These problems include the spectrometers limited resolution, the commonly encountered drifts in output readings, and the fact that all RPD spectrometers produced integral spectra. This last point is particularly significant. Because the ESCA source is so dim, the shot noise is very significant and the signal to noise ratio is quite low. Under these circumstances an integral spectrum will be extremely noisy at the high current end, that is, where the retarding voltage is low. The signal to noise ratio can be improved by long or repeated observations, but the chief advantages of the RPD spectrometer are then lost.

In the Review of Scientific Instruments, Volume 40, published in July of 1969, N. J. Taylor discussed various considerations in the design of a retarding field analyzer based on display LEED optics for detection of Auger electrons. Taylor succeeded in building a spherical RPD spectrometer with resolving power in excess of 300. This represents a notable improvement over most previous such devices, but is still far short of that necessary for ESCA. In addition, the dissertation discussed the problem of shot noise and signal to noise ratio, but concludes onlythat conventionalslitted spectrometers will be preferable under experimental conditions where the signal to noise ratio of an RPD device is too low.

v SUMMARY OFTHEINVEN'IION For the purposes of a clear exposition, the essential features of the present invention can be discussed in two parts. The first resides in the'determination of the proper parameters and conditions for obtaining higher resolution with the spherical RPD spectrometer than had previously been recorded and the construction and testing of sucha device. The second is in the invention of the post-monochromator? concept which solves the above-mentioned signal to noise problem, thus aldim spectrum commonly associated with the ESCA technique, as well as the AES technique.

The proper parameters for generating high resolution spectra with the gridded RPD analyzer were determined byianalyzing the electron trajectories with spe cially'written computer programs. From these analyses we were able to determine the appropriate sample to various physical parameters as functions of the desired resolution will be discussed subsequently.

The second relevant feature of this type of spectrometer is the incorporation of the so-called post monochromator (P.M.) arrangement. The P.M. consists of theelectrostatic system of grid 2, the lateral lowing the use of the RPD spectrometer for the type of Y secondary electron suppressor, the axial secondary I electron suppressor, the pot, or integral current, collector, and the differential current collector. Typically, grid 2 and the axial suppressor grid are connected together. The lateral suppressor is biased several volts .negative so that the three grids together tend tofocus the electron flux on the differential collector. The differential collector in turn is biased positively so as to collect these electrons. The essence of the post monochromator concept is that the focusing operates only on those electrons-that exit from grid 2 with very low kinetic energy. Fast electrons are notfocused, but instead pass through the suppressor grids and are collected on the pot collector. Therefore, since the RPD grid operates as a high pass filter and the post monochromator as a low pass filter, the instrument operates with a composite slit function, and a conventional, as opposed to integral spectrum is generated. The signal to noise problems associated with integral "spectra are thus avoided.

A further advantage of the post monochromator is that it selectivelycollects the slowest electrons passing the second grid, and therefore, 'it increases the resolution of the-system. Essentially, the spherical grids and the post monochromator act as a pair of analyzers in series, and the resolving power of 'the combination is higher than that of either element singly.

All of these improvements coact in concert to produce an electron spectrometer of the type described which has a higher resolution and greater sensitivity than such devices in the prior art, and which yields a differentiated output suitable for accurate chemical analysis by the ESCA technique. In one embodiment a predeceleration grid may be employed in proximity to the electron generating sample source to further retard and disperse the electron flux before reaching the spherical retarding grids, while in another embodiment,

an electron opaque barrier may be positioned in front of and coaxial with the first spherical grid to prevent unwanted high velocity axial traversing electrons from passing directly through the spherical grids and into the post monochromator selective detector.

' OBJECTS OF THE INVENTION Accordingly, a principal objectof the invention is to provide a relatively simple electron spectrometer having higher resolution and greater sensitivity than heretofore available.

An additional object of the invention is to provide an improved electron spectrometer having resolution and sensitivity sufficient to enable chemical analysis of metals and other molecular solids by the techniques of ESCA and/or Auger Electron Spectroscopy.

A further object is to improve the resolution. and sensitivity of retarding potential difference electron spectrometers generally.

A more particular object is to provide an improved post monochromator structure for use with sphericai grid electron analyzers.

Still anoth'er'object is to provide retarding potential difference spectrometers having conventional rather than integrated output spectra.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

I The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view, partially cutaway,

showing the internal structural assembly of apparatus I portion of the apparatus of FIG. 1 showing an alternative embodiment of sample holder construction useful in the invention; 1

FIG. 4 is a partial cross-sectional view of a central portion of the apparatus of FIG. 1, similar to the section of FIG. 2, showing an embodiment of the invention wherein an opaque shadow mask is axially disposed adjacent a first retarding screen;

FIG. 5 is a partial cross-sectional view of the lower portion of the apparatus, similar to FIG. 3, showing an alternative embodiment wherein a pre-deceleration grid is disposed directly above the sample holder;

FIG. 6 is a schematic diagram of an electrical circuit for energizing the apparatus of the invention;

FIG. 6A is a simplified circuit diagram similar to FIG. 6 showing the post monochromator portion of the invention without an electron multiplier output, as adapted for AES or other types of electron spectrosco- W and FIG. 7 is a copy of a recording trace obtained with the apparatus of the invention, showing the form of output information obtainable when the invention is employed for Electron Spectroscopy Chemical Analy- SIS.

Similar reference characters refer to similar structural elements throughout the several Figures of the drawings.

Referring now in greater detail to FIGS. 1 and 2 of the drawings, the apparatus of the invention is hermetically sealed within a ferromagnetic enclosure comprising a cylindrical wall 10, a base plate 11 and a top plate 12. A high vacuum is drawn through an external vacuum pump (not shown) connected to a pipe line 13 communicating with the interior of the enclosure. A hermetically scalable access opening 14 is provided through the outer casing wall to enable the placement of sample materials to be tested onto the surface of a sample holder 15 (FIG. 2) through the opening 16 in cylindrical end 17 of an inverted and truncated metallic cone 18 which is integrally connected with, as by welding or soldering, the cylindrical metallic member 17. The assembled cone 18 and cylinder 17 are rigidly supported by an electrically conductive metallic flange 19 which is securely mounted to the base plate 1 l concentric with the vertical axis of the external ferromagnetic cylinder wall 10. The external metallic structure is grounded, thereby applying ground potential to the inner cylinder 17 and cone 18. An X-ray source or electron gun 20 (FIG. 2) within the sealed enclosure has an exit tube 21 aligned with the opening 16 whereby a beam of mono-energetic X-ray photons is directed upon a sample of material which has been placed upon the surface of sample holder 15.

Referring now more particularly toFIG. 2 of the drawings the remaining internal structure of the apparatus of FIG. 1 will be described in detail. Mounted above the upper open end of cone 18 are a pair of electron retarding grids 24 and which are insulatingly supported by a plurality of upstanding stanchions such as 26 and 27. Grids 24 and 25 are preferably made of stainless steel wire mesh formed as segments of spheres having a common center located on the surface of sample holder 15 at the point where X-ray beam 22 intersects the vertical axis of cone 18. It is desirable that grids 24 and 25 should be transparent to electron streams while being as tightly woven as possible to maintain physical integrity. In practice we have found that screens of 100 mesh per inch woven from 0.002 inch stainless steel wire are quite satisfactory to produce the high degree of resolution required for ESCA. Between grids 24 and 25, also insulatingly mounted on stanchions 26 and 27, are a plurality of concentric metallic rings 28, 29 and 30 which serve to correct field fringing between the differently charged grids 24 and 25. The first spherical grid 24 is electrically connected to the first fringing field ring 28, while the second spherical grid 25 is electrically connected to the uppermost fringing field ring 30. Adjacent fringing field rings 28-30 are interconnected through a plurality of fixed resistance elements as shown schematically in FIG. 6 whereby a uniform field potential gradient is maintained between spherical grids 24 and 25.

-Still referring to FIG. 2, above the second retarding spherical grid 25 is mounted a cylindrical cup shaped collector anode 31 which is supported on an annular insulated bushing 32 affixed to the top of supporting stanchions 26 and 27. Anode 31 may be grounded as shown schematically in FIG. 6 and still remain positive with respect to negatively biased grids 25, 34 and 36. Within the cup shaped anode 31, adjacent to but spaced from the inner cylindrical wall 33 is a cylindrical suppressor grid 34 to which negative potential is applied as shown schematically in FIG. 6. Also within the anode cup 31, adjacent to but spaced from the inner planar surface 35 is a second suppressor grid 36 to which a negative potential of the same potential as grid 25 is applied as shown schematically in FIG. 6. Suppressor grids 34 and 36 are securely held in substantially uniformly spaced relation from the anode walls 33 and 35 by insulated bushing means (not shown). A selective electron collector anode 37, which is preferably formed of a small conically shaped stainless steel cup or funnel as shown in FIG. 2 is insulatingly mounted through a central axial opening in the planar wall 35 of anode 31 and is securely held in position by an insulating bushing 38. The selective electron collector 37 passes through a central axial opening 39 in radial suppressor grid 36 and preferably extends slightly beyond opening 39 as shown in FIG. 2 so that no stray secondary electrons which may escape from anode 31 can be captured by anode 37. The opposite end of collector anode 37 is connected to an electron multiplier indicated generally as 40 in FIG. 2, and the output signals from electron multiplier 40 are insulatingly brought through the outer ferromagnetic casing wall 12 by a hermetically sealed bushing such as 41.

OPERATION OF THE SPECTROMETER The operation of the invention will first bedescribed with reference to FIG. 2 in the practice of Electron Spectroscopy Chemical Analysis. After a sample of material to be analyzed has been placed on the inclined surface of sample holder 15, the access hatch 14 has been securely closed and sealed, and a high vacuum has been pumped, a first level of negative potential is applied to spherical retarding grid 25. Meanwhile the main collector anode 31 is connected to ground, appropriate negative potentials have been applied to suppressor grids 34 and 36, as shown by the schematic circuit diagram of FIG. 6; and the differential electron collector 37 is operated at a positive potential V with respect to the region defined by grids 25, 34 and 36. V, the negative potential applied to retarding grid 25, determines the energy range passed by the spectrometer. In the case of both ESCA and AES, the useful range of V, is between and 1,500 volts. For the typical value of 1,000 volts V, the other potentials are V, 1.5 volts and V,, 300 volts. The multiplier voltage V is determined by the specifications of the electron multiplier. For the Bendix Channeltron, it is 2,800 to 3,000 volts.

When the X-ray tube 20. is energized at approximately 20 KV and 50 ma, a monochromatic X-ray beam 22, usually aluminum Ka or magnesium Ka strikes the 1 sample on the inclined surface of sample holder 15.

- cone 18 or the cylinder 17 are eventually by-passed to ground but a very substantial electron flux, containing many-electrons traveling at different velocities because of the differing energy levels at which they are released from the sample, drift through the substantial conical angle and either impinge upon or pass through grid 24. Those electrons which pass through grid 24 are retarded by the negative field existing in the space X between spherical grids 24 and 25. The higher energy electrons pass through. grid 25 and continue on generally straight courses to impinge upon the main collector anode 31'. However, the electrons with energies slightly in excess of the retarding potential on grid 25 are focused by the electrostatic system consisting of grids 25, 34 and 36 and differential collector 37. The

electrons strike the differential collector and create secondary particles which pass into the electron multiplier, where the flux is greatly amplified and from whence a'signal corresponding to electron counts per second is derived to operate a pen recorder, an oscilloscope or a counter input to a computer.

In cases where the electron flux incident upon the difi'erential collector is so high that an electron multiplier is not necessary, as is frequently the case when performing AES, the differential collector current may be read directly as shown in FIG. 6A.

As the negative potential applied to spherical grid 25 is progressively increased, by the variable power supply V, of FIG. '6, the electron count is recorded at successive kinetic energies and the appearance of this record provides the analytical chemist with detailed information on the molecularstructure of the sample material under test. A typical recording trace obtained from a gold sample with the apparatus described is shown by FIG. 7. The doublet structure shown by the two principal peaks in FIG. 7 is typical of the heavier elements. Each molecular element has a distinctive signature by which it can be positively identified, and the signaelectron collector in the post monochromator structure shown in the upper portion of FIG. 2 makes the invention highly desirable for AES technique as well as for ESCA.

DESCRIPTION OF POST MONOCHROMATQR The post monochromator section of the electron spectrometer illustrated in FIG. 2 is that portion of the apparatus extending above and including grid 25. Thus, it is the combination of spherical grid 25, cylindrical grid 34 and radial planar grid 36 within the cylindrical anode cup 31, and the selective electron collector anode 37 which comprise our unique postmonochromator structure. While grids 34 and 36 both function as secondary electron suppressors, to prevent the emission of secondary electrons from the inner surfaces 33 function for directing the low energy electrons into the selective collector 37.. The electrostatic system of ele- I ments 25, 34, 36 and 37 functions as a weak electron lens focusing only slow electrons on the collector element 37. The fast. electrons are not substantially deflected by this lens and pass through the suppressor grids to be collected on element 31. Thus, the spectrometer operates with a true energy band pass function accepting only those electrons with energy high enough to pass through grid 25, but not sohigh that they are not focused by the post monochromator. It therefore provides a much higher signal to noise ratio for both ABS and ESCA techniques than a retarding grid analyzer does alone. It also enables the selective electron collector anode 37 to be coupled directly to an electron multiplier 40 whereby the otherwise very dim electron spectra is amplified many fold to produce has heretofore been known. When the post monochrotures of chemical compounds provide a great deal of I useful information on the exact manner in which molecules are linked. The peaks correspond to photoelectrons excited from the N and N subshells of gold by aluminum Ka radiation.

Referring again to FIG. 2 of the drawings, the apparatus of the invention may also be used in the technique known as Auger Electron Spectroscopy 1 determined by its ability to detect a low energy, broad Auger peak in a large background of scattered electrons. For this reason the improvement in sensitivity, i.e., signal-to-noise ratio, provided by our differential mator structure is combined with the large area spherical segmental retarding grids 24 and25 separated by the substantial retarding potential field therebetween as maintained by the graduated negative potentials of the fringing field correction rings 28-30, and positioned above the wide conical angle formed by the, conical shield 18 a substantial free-drift distance R, from the test specimen 15, as shown in FIG. 2, the apparatus of the invention provides a relatively economical means for obtaining the very high resolution required for the practice of ESCA.

We have found in laboratory use of the apparatus of the invention for the practice of ESCA technique that maximum resolution and minimum background noise is All internal parts (except insulation) are fabricated of stainless steel For use in AES all of the above dimensional parameters may be reduced proportionally to the order of approximately one third to one-fifth of the given dimensions for ESCA. The essential relationship between the size of specimens, the radius of curvature of the retarding potential grids, and the size or mesh of the retarding potential grids to produce a desired resolving power R may be expressed as follows:

Where R represents resolving power,

R represents radius of curvature of first grid,

R represents radius of curvature of second grid,

d represents diameter of sample holder, and

H8 represents mesh of the retarding grids,

R 113/2 512, and

R z (1M5) pm/a) ALTERNATIVE STRUCTURAL EMBODIMENTS FIG. 3 of the drawings shows an alternative form of test specimen sample holder which is formed as a cone 50 with its apex pointed along the central vertical axis toward the center of the analyzer potential retarding grids. The material to be analyzed is deposited uniformly on the surface of this cone and is irradiated with X-rays for ESCA and with primary electrons for AES. An annular source of X-rays 20 irradiates the entire conical surface of sample holder 50. Theoretical calculations indicate that this conical configuration for the sample holder may produce greater secondary electron flux for any given density of primary radiation whether by X-rays or an electron beam.

FIG. 4 illustrates a refinement of the structure of FIG. 2 wherein an electron opaque shadow mask 51 is suspended by wires 52-52 across the center line of spherical grid 24. The mask 51 stops high velocity general electrons and fluorescent X-rays which may be traveling along the central vertical axis of the system from passing directly through analyzer grids 24 and 25 and into the selective electron differential collector 37. The mask 51 is circular and of a diameter approximately equal to or slightly greater than the outer diameter of collector 37.

FIG. 5 illustrates another alternative form of construction wherein a pre-deceleration grid 54 is mounted in the neck of a conical funnel 55 which is secured to the top of cylindrical enclosure 17 in close proximity to the sample holder which is biased positively with respect to ground. Grid 54 being connected to ground potential serves to provide initial decelerating restraint upon all the secondary electrons emitted from the irradiated test sample and thereby enables a reduction in the length R of the necessary free-drift path between the sample holder 15 and the first spherical grid 24, as well as enabling reduction of the grid spacing X. It is to be understood that the structural refinements represented by the pre-deceleration grid 54 of FIG. 5, the shadow mask 51 of FIG. 4 and the conical sample holder 50 of FIG. 3 may be incorporated singly or in any multiple combination into the structure of FIG. 2, all within the scope of the invention.

Referring now to the schematic diagram of FIG. 6 it will be seen that the conical shield 18, the first spherical grid 24, the first fringing ring 28 and the general electron collector 31 are all connected to ground. A

negative potential V, from a variable voltage source 60 is applied to retarding spherical grid 25, top fringing ring 30 and radial suppressor screen grid 36. A substantially lesser suppressor grid voltage V, provided by source 61 is applied to the cylindrical screen grid 34. Positive potential V is supplied by power source 62 connected between screen 36 and selective electron collector anode 37. As shown in FIG. 6 the conical anode37 is integrally connected to electron multiplier 40 which may be, for example, of the type sold by the Bendix Corporation under the trademark Channeltron. Positive operating potential V for the electron multiplier 40 is provided by source 63' connected through a fixed resistor 64 of 10K ohms. Output pulses from electron multiplier 40 are capacity coupled through a fixed capacitor 65, of 0.001 mfd capacity, to the input of a multichannel counter 66 which may be, for example, the Fabritek Model 1070. The output of pulse counter 66 is connected to a conventional pen recorder 67.

Still referring to FIG. 6, the value of the negative potential V, applied to retarding spherical grid 25 determines the energy range of electrons passed by the spectrometer of the invention. For either Electron Spectroscopy Chemical Analysis or Auger Electron Spectroscopy the useful range of V, is from 100 volts to 1,500 volts. For a typical working value of approximately 1,000 volts V,, the value of V, should be approximately 1.5 volts and the value of V should be approximately 300 volts. The multiplier voltage V,, is as specified by the manufacturer of the particular electron multiplier that is used. For example, for the Bendix Channeltronwhich we have used the voltage V supplied by power source 63 is from 2,800 volts to 3,000 volts.

Reference is now made to FIG. 6A of the drawings which is a simplified schematic of the post monochromator portion of the invention as the same may be employed for Auger Electron Spectroscopy where the use of an electron multiplier is not required. Thus the conical selective electron collector anode 37 in FIG. 6A is directly connected through resistor 64a of approximately one megohm resistance to the positive potential source 62. The voltage values for V,,, V and V, in the circuit of FIG. 6A are the same as those given above with reference to FIG. 6. Because of the substantially greater electron flux density encountered in AES there is no need for an electron multiplier output in the practice of this technique. The voltage drop across resistor 64, which appears on output terminals 68, can be connected directly to an oscilloscope or other recorder.

. FIG. 7 depicts a pen recorded trace of a typical electron spectra developed by the apparatus of the invention, employing the circuit of FIG. 6 for ESCA. The sample was a vacuum evaporated gold film deposited on sample holder 15. The output pulses from electron multiplier 40 were differentiated with respect to the retarding voltage V, while maintaining V,, V,, and V constant with respect to ground potential. The twin peaks 70 and 71, which are typical of the doublet peak spectra found in most metals and heavy molecular elements, in this case (with gold sample) were found to be separated by 3 42 volts with the first peak 70 occurring at 83 i /2 V, volts binding energy and the second peak 71 occurring at 87 i V2 V,. Peak 70 corresponds It will thus be seen that the objects set forth above,

among those made apparent from the preceding description, are efliciently attained and, since certain changes may be made in carrying out the above methods and in the construction set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in alirniting sense. 1

It is also to be understood thatthe following claims are intended to cover all of the generic and specific features of the invention which, as a matter of language, might be said to fall therebetween.

Having describedour invention, what we claim as new and desire to secure by Letters Patent is:

1. In a retarding potential difference electron spec- Y beyond the last retarding grid and substantially concentric with an axis normal to the center of said grid,

B. a secondary electron suppressor grid within said anode cup in proximity to but insulated from the inner surfaces thereof,

1 means applying a negative potential to said suppressor grid,

2. aligned axial apertures formed in the center of said suppressor grid and in the center of anode cup,

C. a differential current collector insulatingly passing through said aligned apertures, and v 1. means applying a positive potential with respect 40 to said secondary electron suppressor to said differential current collector, whereby slower electrons are focused by the secondary electron suppressor and are captured by said differential current collector. 2. The combination of claim 1 wherein said differential current collector includes an electron multiplier. i

3. In a retarding potential difference electron spectrometer having a plurality of retarding grids, a post monochromator comprising in combination,

r C. a lateral secondary electron suppressor grid within said anode. cup in proximity to the closed end thereof and insulated from said coaxial suppressor 1. means applying a positive potential to said lateral grid with respect to said coaxial grid,

2. aligned axial apertures formed in the center of said lateral grid'and in the center of said anode cup,

D. a differential current collector insulatingly passing through said aligned apertures, and

1. means applying a positive potential with respect to the potential of said lateral grid to'said differential current collector whereby slower electrons are captured by said differential current collector.

4. The combination of claim 3 wherein said differential current collector is connected to an electron multiplier to produce an amplified differential output.

5. In a retarding field electron spectrometer the combination comprising,

A. beam source of energy directedto impinge upon. a sample of material'in a vacuum,

I B. a sample holder'intersecting the path of said .energy beam and positioned at an angle to the axis of said beam,

C. a conical electrostatic'shield having its axis of revolution substantially normal to the axis of said beam and in alignment with the point of intersection of said beam with said sample holder,

l. a truncated apex end of said conical shield adjacent to said sample holder whereby electrons emitted from a sample on said sample holder by impingement of said beam thereon are caused to enter said apex end and drift freely through the length of said conical shield along separate paths,

2. a major open end of said conical shield spaced from said truncated apex end,

D. first and second retarding spherical grids positioned beyond the major open end of said conical shield and having a common center of curvature substantially coinciding with the intersection of said beam axis with the axis of said conical shield, 1. said first grid spaced from said sample holder by a ra R1,

2. said second grid further spaced from said sample holder by a radius R, X,

E. a cylindrical cup shaped collector anode coaxial with said conical shield and positioned beyond said second grid by an axial distance Y,

l. a secondary electron repeller screen between said second grid and said anode, in close proximity to said anode, and connected with a source of negative potential, 2. an axial opening through the center of said anode and said repeller screen, and

F. a substantially conically shaped difi'erential current collector insulatingly inserted through said axial opening and connected with an electron multiplier.

6. The combination of claim 5 wherein said energy source produces a substantially monochromatic X-ray beam.

7. The combination of claim 5 wherein said energy source comprises an electron beam gun.

8. In a retarding potential spherical grid electron spectrometer having a resolving power R,

A. a sample holder having a diameter d,,

B. first and second retarding grids each formed of spherical screen segments, having a mesh of l/8),

1. both of said grids having their centers of curvature located at the center of said sample holder,

2. said first grid having a radius of curvature R 3. said second grid having a radius of curvature R C. the relationship between grid radii, grid mesh and sample diameter where:

R approximately equals (d,"/2 [2 R and R approximately equals l/ 42) (R R /8). 9. The combination of claim 5 wherein:

R approximately equals 35 cm X approximately equals 6 cm Y approximately equals 10 cm.

10. The combination of claim 5 including a predeceleration grid interposed between said sample holder and said first retarding spherical grid, said predeceleration grid being mounted in the truncated apex end of said conical electrostatic shield, and the sample holder is biased positively with respect to said predeceleration grid.

1 l. The combination of claim 5 including an electron opaque shadow mask centrally located on the axis of said conical shield between said sample holder and said first retarding spherical grid.

12. The combination of claim 5 wherein the mesh of said first and second retarding spherical grids is more .closely woven and of a larger diameter wire than said secondary electron repeller screen.

13. The combination of claim 12 wherein the mesh of said first and second retarding spherical grids is approximately twice the mesh of said secondary electron repeller screen, and the diameter of said first and second grid wires is approximately twice the diameter of said repeller screen wire.

14. The combination of claim 13 wherein said first and second retarding spherical grids are formed of stainless steel wire approximately 0.002 inch in diameter and of approximately 100 X 100 mesh, and said secondary repeller screen is formed of stainless steel wire approximately 0.001 inch in diameter and of approximately 50 X 50 mesh.

15. The combination of claim 5 wherein said sample holder is formed as a substantially right circular cone of stainless steel coaxial with said conical electrostatic shield and having its apex directed toward the center of said first retarding spherical grid.

16. The combination of claim 5 including a plurality of circular fringing field correction rings having inner diameters substantially equal to the diameters of said first and second retarding spherical grids and substantially greater outer diameters, said correction rings insulatingly mounted in stacked and substantially uniform spaced relation between said first and second spherical grids and coaxial with said conical electrostatic shield and said cylindrical anode cup.

17. The combination of claim 16 wherein the one of said correction rings adjacent to said first retarding spherical grid is electrically connected thereto and the one of said rings adjacent to said second spherical grid is electrically connected thereto.

18. The combination of claim 17 including fixed resistance elements connected between adjacent fringing field correction rings whereby successive rings between said first and second retarding spherical grids are charged with increasing negative potentials.

19. In a retarding field electron spectrometer the combination co sin A. a substariii aiiy r nonochromatrc energy source producing a beam of radiant energy,

B. a sample holder intersecting the beam path of said energy source and positioned at an angle to the axis of said beam,

C. a conical electrostatic shield having its axis of revolution substantially normal to the axis of said energy source beam and in alignment with the point of intersection of said beam with said sample holder,

1. a truncated apex end of said conical shield adjacent to said sample holder whereby electrons emitted from a sample on said sample holder by impingement of said energy beam thereon are caused to enter said apex end and drift freely through the length of said conical shield along separate paths,

2. a major open end of said conical shield spaced from said truncated apex end,

D. first and second retarding spherical grids positioned beyond the major open end of said conical shield and having a common center of curvature substantially coinciding with the intersection of said energy beam axis with the axis of said conical shield,

1. said first grid spaced from said sample holder by a radius R,,

2. said second grid further spaced from said sample holder by a radius R X,

E. a cylindrical cup shaped collector anode coaxial with said conical shield and positioned beyond said second grid by an axial distance Y,

1. a secondary electron repeller screen between said second grid and said anode, in close proximity to said anode and connected with a source of negative potential,

2. an axial opening through the center of said anode and said repeller screen, and

F. a substantially conically shaped differential current collector insulatingly inserted through said axial opening and connected with an electron multiplier.

20. The combination of claim 19 wherein said energy source comprises an electron beam gun.

21. The combination of claim 19 wherein:

R approximately equals 35 cm X approximately equals 6 cm Y approximately equals 10 cm.

22. The combination of claim 19 wherein said energy source comprises an X-ray generator. 

1. In a retarding potential difference electron spectrometer having a plurality of retarding grids, a post monochromator comprising in combination, A. an integral current collector anode cup positioned beyond the last retarding grid and substantially concentric with an axis normal to the center of said grid, B. a secondary electron suppressor grid within said anode cup in proximity to but insulated from the inner surfaces thereof,
 1. means applying a negative potential to said suppressor grid,
 2. aligned axial apertures formed in the center of said suppressor grid and in the center of said anode cup, C. a differential current collector insulatingly passing through said aligned apertures, and
 1. means applying a positive potential with respect to said secondary electron suppressor to said differential current collector, whereby slower electrons are focused by the secondary electron suppressor and are captured by said differential current collector.
 2. aligned axial apertures formed in the center of said suppressor grid and in the center of said anode cup, C. a differential current collector insulatingly passing through said aligned apertures, and
 2. The combination of claim 1 wherein said differential current collector includes an electron multiplier.
 2. a major open end of said conical shield spaced from said truncated apex end, D. first and second retarding spherical grids positioned beyond the major open end of said conical shield and having a common center of curvature substantially coinciding with the intersection of said beam axis with the axis of said conical shield,
 2. said second grid further spaced from said sample holder by a radius R1 + X, E. a cylindrical cup shaped collector anode coaxial with said conical shield and positioned beyond said second grid by an axial distance Y,
 2. an axial opening through the center of said anode and said repeller screen, and F. a substantially conically shaped differential current collector insulatingly inserted through said axial opening and connected with an electron multiplier.
 2. said first grid having a radius of curvature R1,
 2. a major open end of said conical shield spaced from said truncated apex end, D. first and second retarding spherical grids positioned beyond the major open end of said conical shield and having a common center of curvature substantially coinciding with the intersection of said energy beam axis with the axis of said conical shield,
 2. said second grid further spaced from said sample holder by a radius R1 + X, E. a cylindrical cup shaped collector anode coaxial with said conical shield and positioned beyond said second grid by an axial distance Y,
 2. an axial opening through the center of said anode and said repeller screen, and F. a substantially conically shaped differential current collector insulatingly inserted through said axial opening and connected with an electron multiplier.
 2. aligned axial apertures formed in the center of said lateral grid and in the center of said anode cup, D. a differential current collector insulatingly passing through said aligned apertures, and
 3. said second grid having a radius of curvature R2, C. the relationship between grid radii, grid mesh and sample diameter where: R approximately equals (ds2/2 Square Root 2 R12) and R approximately equals (1/ Square Root 2) . (R2 - R1/ delta ).
 3. In a retarding potential difference electron spectrometer having a plurality of retarding grids, a post monochromator comprising in combination, A. an integral current collector anode cup positioned beyond the last retarding grid and substantially concentric with an axis normal to the center of said grid, B. a coaxial secondary electron suppressor grid within said anode cup in proximity to the inner sidewall thereof,
 4. The combination of claim 3 wherein said differential current collector is connected to an electron multiplier to produce an amplified differential output.
 5. In a retarding field electron spectrometer the combination comprising, A. beam source of energy directed to impinge upon a sample of material in a vacuum, B. a sample holder intersecting the path of said energy beam and positioned at an angle to the axis of said beam, C. a conical electrostatic shield having its axis of revolution substantially normal to the axis of said beam and in alignment with the point of intersection of said beam with said sample holder,
 6. The combination of claim 5 wherein said energy source produces a substantially monochromatic X-ray beam.
 7. The combination of claim 5 wherein said energy source comprises an electron beam gun.
 8. In a retarding potential spherical grid electron spectrometer having a resolving power R, A. a sample holder having a diameter ds, B. first and second retarding grids each formed of spherical screen segments, having a mesh of (1/ delta ),
 9. The combination of claim 5 wherein: R1 approximately equals 35 cm X approximately equals 6 cm Y approximately equals 10 cm.
 10. The combination of claim 5 including a pre-deceleration grid interposed between said sample holder and said first retarding spherical grid, said pre-deceleration grid being mounted in the truncated apex end of said conical electrostatic shield, and the sample holder is biased positively with respect to said pre-deceleration grid.
 11. The combination of claim 5 including an electron opaque shadow mask centrally located on the axis of said conical shield between said sample holder and said first retarding spherical grid.
 12. The combination of claim 5 wherein the mesh of said first and second retarding spherical grids is more closely woven and of a larger diameter wire than said secondary electron repeller screen.
 13. The combination of claim 12 wherein the mesh of said first and second retarding spherical grids is approximately twice the mesh of said secondary electron repeller screen, and the diameter of said first and second grid wires is approximately twice the diameter of said repeller screen wire.
 14. The combination of claim 13 wherein said first and second retarding spherical grids are formed of stainless steel wire approximately 0.002 inch in diameter and of approximately 100 X 100 mesh, and said secondary repeller screen is formed of stainless steel wire approximately 0.001 inch in diameter and of approximately 50 X 50 mesh.
 15. The combination of claim 5 wherein said sample holder is formed as a substantially right circular cone of stainless steel coaxial with said conical electrostatic shield and having its apex directed toward the center of said first retarding spherical grid.
 16. The combination of claim 5 including a plurality of circular fringing field correction rings having inner diameters substantially equal to the diameters of said first and second retarding spherical grids and substantially greater outer diameters, said correction rings insulatingly mounted in stacked and substantially uniform spaced relation between said first and second spherical grids and coaxial with said conical electrostatic shield and said cylindrical anode cup.
 17. The combination of claim 16 wherein the one of said correction rings adjacent to said first retarding spherical grid is electrically connected thereto and the one of said rings adjacent to said second spherical grid is electrically connected thereto.
 18. The combination of claim 17 including fixed resistance elements connected between adjacent fringing field correction rings whereby successive rings between said first and second retarding spherical grids are charged with increasing negative potentials.
 19. In a retarding field electron spectrometer the combination comprising, A. a substantially monochromatic energy source producing a beam of radiant energy, B. a sample holder intersecting the beam path of said energy source and positioned at an angle to the axis of said beam, C. a conical electrostatic shield having its axis of revolution substantially normal to the axis of said energy source beam and in alignment with the point of intersection of said beam with said sample holder,
 20. The combination of claim 19 wherein said energy source comprises an electron beam gun.
 21. The combination of claim 19 wherein: R1 approximately equals 35 cm X approximately equals 6 cm Y approximately equals 10 cm.
 22. The combination of claim 19 wherein said energy source comprises an X-ray generator. 